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| United States Patent |
5,631,062
|
|
Masuyama
, et al.
|
May 20, 1997
|
Magnetic recording medium
Abstract
A magnetic recording medium comprising a nonmagnetic support and a magnetic
layer, wherein the magnetic layer is provided on one surface of the
nonmagnetic support, comprises a ferromagnetic particle, and has a surface
roughness of from 2 to 10 nm; the nonmagnetic support has plural layers;
at least one layer of the plural layers comprises an antistatic agent; and
the nonmagnetic support has a surface roughness of from 5 to 30 nm on the
opposite surface to the surface on which the magnetic layer is provided.
| Inventors:
|
Masuyama; Kenichi (Kanagawa, JP);
Takano; Hiroaki (Kanagawa, JP);
Kato; Kazuo (Kanagawa, JP);
Hanai; Kazuko (Kanagawa, JP);
Hibino; Noburo (Kanagawa, JP)
|
| Assignee:
|
Fuji Photo Film Co., Ltd. (Kanagawa, JP)
|
| Appl. No.:
|
248246 |
| Filed:
|
May 24, 1994 |
Foreign Application Priority Data
| Current U.S. Class: |
428/141; 428/143; 428/845.4; 428/900 |
| Intern'l Class: |
G11B 005/66; G11B 005/70 |
| Field of Search: |
428/694 ST,694 SL,694 SG,694 BS,694 BN,695 BR,694 B,141,143,900
|
References Cited
U.S. Patent Documents
| 4590119 | May., 1986 | Kawakami et al. | 428/216.
|
| 4613520 | Sep., 1986 | Dasgupta | 427/128.
|
| 4664975 | May., 1987 | Kobayashi et al. | 428/323.
|
| 4687700 | Aug., 1987 | Hensel et al. | 428/213.
|
| 5206084 | Apr., 1993 | Takeda et al. | 428/336.
|
| 5281472 | Jan., 1994 | Takahashi et al. | 428/336.
|
| 5318823 | Jun., 1994 | Utsumi et al. | 428/143.
|
| 5366783 | Nov., 1994 | Utsumi et al. | 428/141.
|
| Foreign Patent Documents |
| 58-71922 | Apr., 1983 | JP.
| |
| 63-308059 | Dec., 1988 | JP.
| |
Primary Examiner: Thibodeau; Paul J.
Assistant Examiner: Sand; Stephen
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
What is claimed is:
1. A magnetic recording medium comprising an organic or polymeric
nonmagnetic support and a magnetic layer, wherein
the magnetic layer is provided on one surface of the nonmagnetic support,
comprises a ferromagnetic particle, and has a center line average surface
roughness of from 2 to 10 nm;
the nonmagnetic support has plural layers;
at least one layer of the plural layers comprises an antistatic agent
selected from the group consisting of polyalkylene glycol, sulfonic acid
metal salt derivative, aromatic amine, ammonium salt of aromatic amine,
and a mixture of an aromatic amine and an ammonium salt of an aromatic
amine;
the antistatic agent is incorporated in at least the surface layer of the
nonmagnetic support which is opposite to the magnetic layer; and
said surface layer has a center line average surface roughness of from 5 to
30 nm.
2. The magnetic recording medium of claim 1, wherein the ratio of the
layers of the nonmagnetic support not subject to antistatic treatment to
the layers of the nonmagnetic support subjected to antistatic treatment is
from 9.5/0.5 to 3/7.
3. A magnetic recording medium comprising an organic or polymeric
nonmagnetic support, a lower layer and a magnetic layer, wherein
the lower layer is provided on one surface of the nonmagnetic support and
comprises an inorganic particle and carbon black;
the magnetic layer is provided on the lower layer, comprises a
ferromagnetic particle, and has a center line average surface roughness of
from 2 to 10 nm;
the nonmagnetic support has plural layers;
at least one layer of the plural layers comprises an antistatic agent
selected from the group consisting of polyalkylene glycol, sulfonic acid
metal salt derivative, aromatic amine, ammonium salt of aromatic amine and
a mixture of an aromatic amine and an ammonium salt of an aromatic amine;
the antistatic agent is incorporated in at least the surface layer of the
nonmagnetic support which is opposite to the magnetic layer; and
said surface layer has a center line average surface roughness of from 5 to
30 nm.
4. The magnetic recording medium as claimed in claim 3, wherein the
ferromagnetic particle is a ferromagnetic alloy particle.
5. The magnetic recording medium of claim 3, wherein the ratio of the
layers of the nonmagnetic support not subject to antistatic treatment to
the layers of the nonmagnetic support subjected to antistatic treatment is
from 9.5/0.5 to 3/7.
Description
FIELD OF THE INVENTION
The present invention relates to a so-called backless magnetic recording
medium comprising a nonmagnetic support composed of plural layers and
having no back coating layer, and particularly to a magnetic recording
medium high in recording density and high in reliability.
BACKGROUND OF THE INVENTION
With recent developments in magnetic recording, the magnetic recording
media which can reproduce images and sounds of higher quality have been
strongly demanded. In order to meet these demands, a reduction in the
particle size of the ferromagnetic particles and an increase in the
density of the magnetic recording media have hitherto been promoted.
Further, since the magnetic recording media are consumed in large amounts,
they have been required to be produced at a lower cost. One technique to
meet these demands is to provide a plurality of the magnetic layers. This
technique is advantageous for increasing the density in that
shorter-wavelength recording characteristics can be imparted to an upper
layer and longer-wavelength recording characteristics can be imparted to a
lower layer, thereby using suitable ferromagnetic particles in the
respective layers. In the use for shorter-wavelength recording alone, the
upper magnetic layer must be decreased in thickness and a nonmagnetic
particle must be used in the lower layer, thereby diminishing
self-demagnetization, which advantageously results in a higher-density
recording medium. At the same time, the recording medium having plural
magnetic layers has a feature that the medium can be produced at a low
cost because suitable materials can be used depending upon the respective
layers. Recently, methods for improving the surface properties of the
magnetic layers, or ferromagnetic particles high having output and low
noise have been required to improve electromagnetic characteristics.
On the other hand, such electromagnetic characteristics of the magnetic
recording media should be evaluated on the condition that the magnetic
recording media have good running ability above a certain level. In order
to secure good running ability, therefore, the surfaces of the magnetic
layers and the backing layers of the magnetic recording media have been
required to be low in the coefficient of friction. For electrification
which is the main cause of drop out or an output defect, it has been
necessary that the magnetic recording media are hard to be charged.
That is, the requirements for the present magnetic recording media are as
follows:
(1) the magnetic recording media have excellent electromagnetic
characteristics;
(2) the magnetic recording media are hard to be charged;
(3) the magnetic recording media have excellent running durability; and
(4) the magnetic recording media have excellent productivity.
First, a surface of the magnetic layer is required to be made as smooth as
possible (for example, JP-A-57-130234 (the term "JP-A" as used herein
means an "unexamined published Japanese patent application") and
JP-A-61-168124). For this purpose, a base surface on which the magnetic
layer is formed is required to be very smooth. In particular, when the
magnetic layers are formed in a multiple-layer structure, a surface of a
nonmagnetic support on which the magnetic layers are formed influences the
surface properties of the lower magnetic layer, which further influences
the surface properties of the upper magnetic layer. Accordingly, the
surface properties of the surface of the nonmagnetic support on which the
magnetic layers are formed is important, and the surface is required to be
smooth. Further, in order to obtain excellent electromagnetic
characteristics, magnetic substances are required to be high in output and
low in noise.
Next, electrification of a magnetic recording tape induced by sliding the
tape on a loading guide for the tape and a cylinder in a VTR or on
cassette members in a cassette causes easy adhesion of a powder falling
from the tape itself, a powder produced from the cassette members or a
surface of the tape by wear due to the contact of the tape with the
cassette members and dust from the outside to the tape, which results in
partial space loss on recording and reproduction of a magnetic head,
leading to a drop-out failure. For the purpose of preventing this, a
reduction in the surface electrical resistance of the surfaces of the
magnetic layers (for example, JP-A-59-16140 and JP-A-59-63029) and a
reduction in the surface electrical resistance of the back coating layers
formed on the surfaces opposite to the surfaces of the nonmagnetic
supports on which the magnetic layers are formed (for example,
JP-A-57-150132 and JP-A-59-3722) have been known. However, the formation
of the back coating layers requires the step of forming the back coating
layers and the development of back coating solutions, resulting in a high
production cost, and brings about powder dropping and separation of the
back coating layers on repeated use, resulting in deteriorated durability.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a magnetic
recording medium which is excellent in electromagnetic characteristics,
low in the coefficient of friction of the back surface, little in
generation of scratches and abrasion particles by repeated running, and
low in electrification, leading to little drop out produced by adhesion of
dust to the medium.
Another object of the present invention is to provide a magnetic recording
medium which is high in recording density and high in reliability.
In order to attain the above-described objects, the present inventors have
conducted intensive investigations into magnetic recording media having no
back coating layer, namely magnetic recording media in which nonmagnetic
supports themselves constitute back surfaces, particularly into the layer
constitution of the nonmagnetic supports of the magnetic recording media,
the antistatic treatment thereof and the surface roughness thereof.
These and other objects of the present invention can be achieved by a
magnetic recording medium comprising a nonmagnetic support and a magnetic
layer, wherein the magnetic layer is provided on one surface of the
nonmagnetic support, comprises a ferromagnetic particle, and has a surface
roughness of from 2 to 10 nm; the nonmagnetic support has plural layers;
at least one layer of the plural layers comprises an antistatic agent; and
the nonmagnetic support has a surface roughness of from 5 to 30 nm on the
opposite surface to the surface on which the magnetic layer is provided.
Preferably, these and other objects of the present invention can be
achieved by the above-described magnetic recording medium, wherein the
antistatic agent is at least one selected from the following (1), (2) and
(3):
(1) polyalkylene glycol;
(2) sulfonic acid metal salt derivative; and
(3) at lease one of aromatic amine and ammonium salt thereof.
Further, these and other objects of the present invention can be achieved
by a magnetic recording medium comprising a nonmagnetic support, an lower
layer and a magnetic layer, wherein the lower layer is provided on one
surface of the nonmagnetic support and comprises an inorganic particle and
carbon black; the magnetic layer is provided on the lower layer, comprises
a ferromagnetic particle, and has a surface roughness of from 2 to 10 nm;
the nonmagnetic support has plural layers; at least one layer of the
plural layers comprises an antistatic agent; and the nonmagnetic support
has a surface roughness of from 5 to 30 nm the opposite surface to the
surface on which the magnetic layer is provided.
Further preferably, these and other objects of the present invention can be
achieved by the above-described magnetic recording medium, wherein the
antistatic agent is at least one selected from the following (1), (2) and
(3):
(1) polyalkylene glycol;
(2) sulfonic acid metal salt derivative; and
(3) at lease one of aromatic amine and ammonium salt thereof; and the
ferromagnetic particle is a ferromagnetic alloy particle.
DETAILED DESCRIPTION OF THE INVENTION
One of the features of the present invention is that the magnetic layer has
a surface roughness of from 2 to 10 nm, and that the nonmagnetic support
has a surface roughness of from 5 to 30 nm on the opposite surface to the
surface on which the magnetic layer is provided. The smoother the surface
of the magnetic layer is, the more improved electromagnetic
characteristics such as output the magnetic recording medium has. On the
other hand, it is known that the surface of the nonmagnetic support
opposite to the surface on which the magnetic layer is provided is
preferably as smooth as possible, because the unevenness of the surface of
the nonmagnetic support is transferred to the surface of the magnetic
layer, when the magnetic layer is brought into contact with the surface of
the nonmagnetic support opposite to the surface on which the magnetic
layer is provided, for example, when the magnetic recording medium is
stored in the overlapped state. However, if the surface is too smooth, the
tape is damaged by sliding on a guide on running of a VTR, or the guide is
stained. Edge damage is also likely to take place.
Then, another feature of the present invention is that the antistatic agent
is directly incorporated in the nonmagnetic support without forming a back
coating layer on the surface of the nonmagnetic support, thereby
exhibiting the antistatic effect while maintaining the original high
durability of the nonmagnetic support. Namely, the surface properties of
the nonmagnetic support are improved to increase the friction coefficient.
Even if the surface of the support is subject to strong forces, no scratch
is produced because of its high durability. When a powder is generated or
floats in the air, it does not adhere to the magnetic recording medium.
Accordingly, the dropout after repeated running is not deteriorated. On
running of the VTR, therefore, the tape is not damaged by sliding on the
guide, or the guide is not stained. Also, edge damage does not take place.
A further feature of the present invention is that the above-described
support contains at least one of the following (1) to (3) as the
antistatic agent:
(1) polyalkylene glycol;
(2) sulfonic acid metal salt derivative; and
(3) aromatic amine and/or ammonium salt thereof.
The polyalkylene glycol shown in (1) is an antistatic agent, and also has
lubricity. The sulfonic acid metal salt derivative shown in (2) and the
aromatic amine and/or the ammonium salt thereof shown in (3) are powerful
antistatic agents. These compounds exhibit their effect when used alone.
However, they are preferably used in combination to obtain a support more
excellent in antistatic property and lubricity.
Thus, the present invention is characterized in that above-described
nonmagnetic support has plural layers, and that at least one of the
above-described layers contains the antistatic agent. The antistatic agent
may therefore be comprised in all of the layers or some of them. When the
antistatic agent is incorporated in some of the layers, it is preferably
comprised in at least the surface layer of the nonmagnetic support
opposite to the surface on which the magnetic layer is provided. This is
because addition of the antistatic agent to at least a place at which the
antistatic effect is desired to be exhibited, namely to the surface layer
of the nonmagnetic support, results in sufficient exhibition of the
antistatic effect at a minimized amount of the antistatic agent used. When
the antistatic agent is comprised in all the layers of the nonmagnetic
support, the guide is likely to be stained on running of the VTR, and edge
damage is also likely to take place. For the layer structure of the
support, therefore, the ratio of the layers not subjected to antistatic
treatment to the layers subjected thereto is preferably from 9.5/0.5 to
3/7, and more preferably from 9/1 to 5/5.
Examples of the nonmagnetic support used in the present invention include a
polyethylene-2,6-naphthalate film (PEN), a 4-6 nylon aramide film, and a
PBO film, in particular, a polyester film. Such polyester film can be
obtained by polycondensing an aromatic dicarboxylic acid (e.g.,
terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid) or
ester thereof with glycol (e.g., ethylene glycol, diethylene glycol,
tetramethylene glycol, neopentyl glycol).
The polyester is also obtained by conducting ester interchange reaction
between aromatic dicarboxylic acid dialkyl ester and glycol, followed by
polycondensation, or by polycondensing aromatic dicarboxylic acid ester as
well as by direct polycondensation of an aromatic dicarboxylic acid with
glycol. Typical examples of such polymer include polyethylene
terephthalate and polyethylene naphthalate.
Examples of the polyalkylene glycol used in the present invention include
polyethylene glycol, polypropylene glycol and polytetramethylene glycol
each having an average molecular weight of 400 or more. Polyethylene
glycol having an average molecular weight of from 1,000 to 20,000 is
preferred. The polyalkylene glycol is added to the nonmagnetic support
film, such as a polyethylene-2,6-naphthalate film (PEN), a 4-6 nylon
aramide film and a PBO film, preferably in an amount of from 0.1 to 20
parts by weight, more preferably from 0.5 to 10 parts by weight, based on
100 parts by weight of the nonmagnetic support film. When the polyalkylene
glycol is added in an amount of less than 0.1 part by weight, it is not
preferred because scratches due to a guide pole on repeated running are
likely to generate. On the other hand, when it is added in an amount of
more than 20 parts by weight, it is not preferred because mechanical
properties and heat resistance are deteriorated.
Examples of the sulfonic acid metal salt derivative used in the present
invention include an organic sulfonic acid metal salt having a functional
group forming an ester bond (e.g., sodium 5-sulfoisophthalate, potassium
5-sulfoisophthalate, sodium
3,5-di(carbo-.beta.-hydroxyethoxy)benzenesulfonate) and an alkyl aromatic
sulfonic acid metal salt (e.g., sodium nonylbenzenesulfonate, sodium
dodecylbenzenesulfonate, sodium decylbenzenesulfonate, sodium
stearylbenzenesulfonate, sodium octylnaphthalenesulfonate, sodium
nonylnaphthalenesulfonate, sodium dodecylnaphthalenesulfonate, potassium
nonylnaphthalenesulfonate, potassium dodecylnaphthalenesulfonate,
potassium stearylnaphthalenesulfonate). Of these, sodium
dodecylbenzenesulfonate and sodium 5-sulfoisophthalate are preferably
used. The sulfonic acid metal salt derivative is added preferably in an
amount of from 0.05 to 20 parts by weight, more preferably from 0.1 to 10
parts by weight, based on 100 parts by weight of the nonmagnetic support
film. When the sulfonic acid metal salt derivative is added in an amount
of less than 0.05 part by weight, it is not preferred because scratches
due to the guide pole on repeated running are likely to generate. On the
other hand, when it is added in an amount of more than 20 parts by weight,
it is not preferred because mechanical properties are deteriorated.
The aromatic amine used in the present invention is preferably a tertiary
amine. Examples thereof include N,N-dimethylaniline, N,N-diethylaniline,
N,N-dibutylaniline, N,N-dibenzylaniline, N,N-diethyl-o-toluidine,
N,N-diethyl-m-toluidine, N,N-diethyl-p-N-methyldiphenylamine,
N,N-benzyl-N-ethylaniline, triphenylamine and
N,N,N,N-tetramethyl-p-phenylenediamine. Examples of the ammonium salt
thereof used in the present invention include salts of the above-described
amines with a fatty acid such as carbonic acid and acetic acid, or with a
hydrohalogenic acid. Of these, triphenylamine and
N,N,N,N-tetramethyl-p-phenylenediamine are preferred. The aromatic amine
and/or salt thereof are added preferably in an amount of from 0.01 to 2.0
parts by weight, more preferably in an amount of from 0.01 to 1.0 part by
weight, based on 100 parts by weight of the nonmagnetic support film. When
the aromatic amine and/or salt thereof are used in an amount of less than
0.01 part by weight, it is not preferred because scratches due to the
guide pole on repeated running are generated. On the other hand, they are
added in an amount of more than 2.0 parts by weight, it is not preferable
because mechanical properties are deteriorated.
These antistatic agents are added to the surface layer of the nonmagnetic
support opposite to the magnetic layers, whereby the anti-scratching
property to the guide pole on repeated running, in addition to the
antistatic property, can be improved. This is probably because the film
stiffness of the nonmagnetic support can be appropriately softened, which
results in decreased scratches against scratching forces on the surface of
the nonmagnetic support when it is scratched on the guide pole.
In the film used as the nonmagnetic support in the present invention, it is
preferred that the surface layer of the nonmagnetic support opposite to
the surface in contact with the magnetic layer has a specified number of
projections having a specified height on the film surface, particularly
preferably by addition of particles of CaCO.sub.3, SiO.sub.2, Al.sub.2
O.sub.3 and an organic filler. The projections improve the durability and
the anti-scratching property of the tape.
In general, the larger size and the larger number of the projections formed
on the film surface by addition of CaCO.sub.3, SiO.sub.2 and the organic
filler have, the lower friction coefficient to improve the anti-scratching
property the magnetic recording medium has. However, the surface roughness
(Ra) is reduced to deteriorate the electromagnetic characteristics.
The height and the number of the projections can be controlled by the
particle size and the amount of CaCO.sub.3 or SiO.sub.2 to be added. The
size of CaCO.sub.3, SiO.sub.2 and the organic filler is preferably from
0.3 to 0.8 .mu.m. A size of less than 0.3 .mu.m thereof provides
insufficient durability, whereas a size of more than 0.8 .mu.m thereof
results in inferior surface properties to deteriorate the electromagnetic
characteristics. The particle size of Al.sub.2 O.sub.3 is preferably 0.2
.mu.m or less, and more preferably 0.1 .mu.m or less. A size of more than
0.2 .mu.m thereof reduces the anti-scratching effect of the base film.
The friction coefficient of the surface of the nonmagnetic support,
particularly the polyester film, of the magnetic recording medium is
preferably 0.30 or less.
The polyester film used in the present invention may be one in which a
layer containing the above-described inorganic particles is provided on at
least one side of a biaxial oriented thermoplastic film.
Further, the above-described nonmagnetic support has plural layers, and a
lubricating agent is provided on the surface layer of the nonmagnetic
support opposite to the surface on which the above-described magnetic
layer is provided, thereby giving the effect of preventing scratches on
the tape support layer and powder dropping therefrom on repeated running.
Examples of the above-described lubricating agent include a fatty acid,
fatty acid ester, a silicon compound and a fluorine lubricating agent.
The magnetic recording medium of the present invention can be produced, for
example, in the following manner. The lower inorganic particle layer is
formed on the nonmagnetic support, followed by magnetic field orientation
treatment, drying treatment and calendering treatment, if necessary. Then,
the upper magnetic layer is formed thereon, followed by magnetic field
orientation treatment, drying treatment and calendering treatment, thus
forming the magnetic layer-coated film.
However, a method called the "wet-on-wet coating system" (disclosed in U.S.
Pat. No. 4,844,946) is particularly preferred in which the upper magnetic
layer is provided on the lower inorganic powder layer concurrently or
sequentially while the lower layer is still in a wet state.
The lower inorganic powder layer in the magnetic recording medium of the
present invention is a layer in which inorganic particles are dispersed in
binders. The inorganic particles used in the present invention include
known ferromagnetic particles such as .gamma.-Fe.sub.2 O.sub.3,
Co-containing FeOx (1.33.ltoreq.x.ltoreq.1.48), FeOx
(1.33.ltoreq.x.ltoreq.1.48), CrO.sub.2, a Co--Ni--P alloy and an
Fe--Co--Ni alloy, and nonmagnetic powders such as TiO.sub.2, BASO.sub.4,
ZnO.sub.2, .alpha.-Fe.sub.2 O.sub.3 and carbon black. They may be used
alone or in combination.
In particular, the ferromagnetic particles or the nonmagnetic particles can
be used properly as the inorganic particles for the upper layers,
depending upon the requirement and use of a magnetic recording system.
The upper magnetic layer in the magnetic recording medium of the present
invention is preferably a layer in which ferromagnetic alloy particles are
dispersed in binders. The ferromagnetic alloy particles used in the
present invention include alloys having at least 75% by weight of metal
ingredients, 80% by weight or more of the metal ingredients being at least
one ferromagnetic metal Or alloy (for example, Fe, Co, Ni, Fe--Co, Fe--Ni,
Co--Ni, Co--Ni--Fe), and the metal ingredients being capable of containing
other components (for example, Al, Si, S, Sc, Ti, V, Cr, Mn, Cu, Zn, Y,
Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd,
B, P) within a range of less than 20% by weight of the metal ingredients;
and iron nitride. The ferromagnetic alloy particles preferably contain Al
or Si or both. Further, the above-mentioned ferromagnetic metals may
contain a small amount of water, hydroxides or oxides. Methods for
producing these ferromagnetic metal particles are already known, and the
ferromagnetic alloy particles, typical examples of the ferromagnetic
particles used in the present invention, can be produced according to
these known methods.
That is, examples of the methods for producing the ferromagnetic alloy
particles include the following methods:
(a) a method in which a complex organic acid salt (mainly an oxalic acid
salt) is reduced with a reducing gas such as hydrogen;
(b) a method in which iron oxide is reduced with a reducing gas such as
hydrogen to obtain Fe or Fe--Co particles;
(c) a method in which a metal carbonyl compound is thermally decomposed;
(d) a method in which a reducing agent such as sodium borohydride,
hypophosphate or hydrazine is added to an aqueous solution of a
ferromagnetic metal to conduct reduction;
(e) a method in which a ferromagnetic metal particle is electrolytically
precipitated by the use of a mercury cathode, followed by separation from
mercury; and
(f) a method in which a metal is vaporized in a low-pressure inert gas to
obtain a fine particle.
When the ferromagnetic alloy particle is used, the shape thereof is not
particularly limited. However, acicular, granular, dice-like, polyhedral
and plate-like particles are usually used. The metal particles obtained by
these methods suffer unfavorable rapid oxidation on contact with the air.
It is therefore preferred to use gradual oxidation methods in which oxide
layers are formed on surfaces of the powders for stabilization. Such
methods include a method in which a metal particle is immersed into an
organic solvent in an inert gas, followed by evaporation of the solvent
and drying in the air, and a method in which a mixed gas of an oxygen gas
and an inert gas having a low oxygen partial pressure is introduced in an
inert gas to increase the oxygen pressure gradually, and finally the air
is passed.
The coercive force of the ferromagnetic alloy particle in the most upper
layer is preferably from 600 to 5,000 Oe (oersted), and more preferably
1,000 to 2,500 Oe. A coercive force of less than 600 Oe results in an
unfavorable deterioration of electromagnetic characteristics in a short
wavelength region, whereas a coercive force of more than 5,000 Oe makes it
impossible to record with a normal head. Further, it is preferred that the
ferromagnetic alloy particle has a specific surface area in accordance
with the BET method of 40 m.sup.2 /g or more, an average longer diameter
of 0.25 .mu.m or less, an acicular ratio of 12 or less and a crystallite
size of from 150 to 300 angstroms. Without the ranges described above,
noise is unfavorably made loud, and a C/N ratio is unfavorably lowered.
Furthermore, the saturation magnetization (.sigma..sub.s) is preferably
from 100 to 160 emu/g.
The thickness of the most upper layer is preferably 1 .mu.m or less, and
more preferably 0.8 .mu.m or less. When the thickness of the most upper
layer is more than 1 .mu.m, the effect of the multiple-layer structure is
unfavorably reduced.
In the present invention, abrasive is preferably used. Although the
abrasive to be used is not particularly limited, abrasive having a Mohs'
hardness of 6 or more, preferably 8 or more, are employed. Examples
thereof include MgO (Mohs' hardness: 6), Cr.sub.2 O.sub.3 (Mohs' hardness:
8.5), .alpha.-Al.sub.2 O.sub.3 (Mohs' hardness: 9), .gamma.-Al.sub.2
O.sub.3 (Mohs' hardness: 7 to 8) and SiC (.alpha. or .beta., Mohs'
hardness: 9.5). The particle size thereof is preferably from 0.01 to 1.50
.mu.m, and more preferably 0.1 to 0.8 .mu.m. The abrasive content is
preferably from 0.3 to 10 parts by weight, more preferably from 0.5 to 5
parts by weight, based on 100 parts by weight of the ferromagnetic
particle. The abrasives different in kind or particle size may be used as
mixtures if desired. It is particularly preferred that the above-described
abrasives are used in the upper magnetic layer.
As a means for reducing the electrification of a magnetic layer, there is
the method of decreasing the electric resistance of the magnetic layer by
adding carbon black to the magnetic layer as described above. However,
carbon black is a nonmagnetic particle. Accordingly, when it is added in
an increased amount, the magnetic characteristics are lowered, resulting
in deteriorated electromagnetic characteristics. In the case of magnetic
layer having the multiple-layer structure, however, an increased amount of
carbon black can be added only to the lower inorganic particle layer, and
a minimized amount of carbon black or no carbon black can be added to the
upper magnetic layer, thereby compatibly preventing the electromagnetic
characteristics from being deteriorated and lowering the electrification.
Examples of the carbon black used in the present invention includes
furnace for rubbers, thermal for rubbers, coloring black and acetylene
black. The average particle size of the carbon black used in the present
invention is preferably from 5 to 100 m.mu., and more preferably 5 to 50
m.mu.. An average particle size of more than 100 m.mu. results in the
insufficient effect of lowering the surface electric resistance. The
carbon black is added preferably in an amount of from 5 to 50% by weight
based on the ferromagnetic powder. When the carbon black is added in an
amount of less than 5% by weight, the effect of lowering the surface
electric resistance is insufficient. When the carbon black is added in an
amount of more than 50% by weight, the electromagnetic characteristics are
deteriorated. Such carbon black is commercially available under the trade
names of VULCAN XC-72, BP905 and BP800 manufactured by Cabot Co., Ltd.;
CONDUCTEX SC manufactured by Colombia Carbon Co., Ltd.; Asahi #50, #55,
#70 and #80 manufactured by Asahi Carbon Co., Ltd.; and 950B, 3250B and
650B manufactured by Mitsubishi Kasei Corporation.
A binder solution for producing a magnetic coating composition used in the
present invention comprises a resin component and a solvent, and further
comprises other additives such as a lubricating agent, if desired.
Examples of the resin component include a thermoplastic resin, a
thermosetting resin, a reactive resin and a mixture thereof which are
conventionally used. Specific examples of the resin component include a
vinyl chloride copolymer (for example, vinyl chloride-vinyl acetate
copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl
chloride-vinyl acetate-acrylic acid copolymer, vinyl chloride-vinylidene
chloride copolymer, vinyl chloride-acrylonitrile copolymer, ethylene-vinyl
acetate copolymer, epoxy group-introduced vinyl chloride copolymer), a
cellulose derivative (for example, nitrocellulose resin, acrylic resin,
polyvinyl acetal resin, polyvinyl butyral resin, epoxy resin, phenoxy
resin, polyurethane resin, polycarbonate polyurethane resin). It is
preferred that these resins contain polar groups such as --COOH,
--SO.sub.3 Na, --OSO.sub.3 H, --SO.sub.2 Na, --PO.sub.3 Na.sub.2 and
--OPO.sub.3 H.sub.2.
When curing agents are used, a polyisocyanate compound is generally used.
The polyisocyanate compound is selected from the compounds generally used
as a curing component for polyurethane resins.
Further, when curing treatment is conducted by electron beam irradiation,
the compound having a reactive double bond (for example, urethane
acrylate) can be used.
Examples of the solvent used for the production of the magnetic coating
composition include ketone such as acetone, methyl ethyl ketone, methyl
isobutyl ketone, ethyl isobutyl ketone and cyclohexanone; ester such as
methyl acetate, ethyl acetate, butyl acetate and glycol acetate monoethyl
ether; ether such as ether, glycol dimethyl ether and dioxane; aromatic
hydrocarbon such as benzene, toluene and xylene; and chlorinated
hydrocarbon such as methylene chloride, ethylene chloride, carbon
tetrachloride, chloroform, ethylene chlorohydrin and dichlorobenzene.
These solvents can be used alone or in combination. Polar solvents such as
ketone or solvents containing polar solvents are particularly preferred.
In producing the magnetic coating composition, the ferromagnetic particle
is homogeneously kneaded with the binder solution to disperse the powder
therein. For this dispersion by kneading, methods are generally utilized
in which the powder is pre-dispersed by the use of a two-roll mill, a
three-roll mill, an open kneader, a pressure kneader or a continuous
kneader, and then after-dispersed by the use of a sand grinder or a ball
mill.
Any of various additives such as lubricating agents and dispersing agents
may be added, of course, to the magnetic coating composition according to
the conventional techniques depending on their purpose.
Examples of the method Which can be utilized for applying the magnetic
coating composition include air doctor coating, blade coating, rod
coating, extrusion coating, air knife coating, squeeze coating,
impregnation coating, reverse roll coating, transfer roll coating, gravure
coating, kiss coating, cast coating, spray coating and spin coating. For
the details of the coating methods, The Newest Coating Techniques
published by Sogo Gijutsu Center, Japan, can be referred to.
In particular, preferred examples of the apparatus and methods for coating
the magnetic recording medium having the double-layer constitution include
extrusion coating methods described in JP-B-1-46186 (the term "JP-B" as
used herein means an "examined Japanese patent publication"),
JP-A-62-95174, JP-A-63-88080, JP-A-60-238179, JP-A-1-236968, JP-A-2-17971,
JP-A-2-26567, JP-A-2-174965 and JP-A-2-265672.
The methods for producing the magnetic recording media of the present
invention have hereinbefore been described for the double-layer system
composed of the upper and lower layers. However, the number of the
magnetic layers may be three or more as a whole, as long as the magnetic
layers of the double-layer structure having the properties specified above
are contained.
The magnetic layers formed on the support by such methods are subjected to
treatment for orientating the ferromagnetic particles contained in the
layers to desired directions with drying if necessary, and the formed
magnetic layers are dried. At this time, the support is usually
transferred at a speed of from 10 to 1,000 m/minute and at a drying
temperature of from 20.degree. to 130.degree. C. Then, the resulting
products are subjected to surface smoothing treatment and cut into desired
shapes if necessary to produce the magnetic recording medium of the
present invention. In these producing methods, it is preferred that the
steps of surface treatment of the fillers, kneading, dispersing, coating,
heat treatment, calendering, radiation (EB) treatment, surface polishing
treatment and cutting are continuously conducted. Further, this may be
divided into several steps as required. In these steps, the temperature is
from 10.degree. to 130.degree. C. and the humidity is from 5 to 20
mg/m.sup.3 when represented by the water content in the air.
The present invention is now illustrated in greater detail by reference to
the following examples which, however, are not to be construed as limiting
the invention in any way. In the following examples, parts are by weight,
unless otherwise indicated.
EXAMPLES
Example 1
Coating Solution for Lower Layer
Ferromagnetic Particle
______________________________________
Co-.gamma.FeO.sub.x 100 parts
(x = 1.37, Hc = 800 Oe, S.sub.BET = 33 m.sup.2 /g)
______________________________________
Binders
______________________________________
Vinyl Chloride Copolymer 10 parts
(MR-110: containing an epoxy group
and an --SO.sub.3 Na group, manufactured
by Nippon Zeon Co., Ltd.)
Polyurethane 5 parts
(UR-8300: containing an --SO.sub.3 Na
group, manufactured by Toyobo
Co., Ltd.)
______________________________________
Curing Agent
______________________________________
Polyisocyanate 5 parts
(Coronate L-75: manufactured by
Nippon Polyurethane Co., Ltd.)
______________________________________
Additives
______________________________________
Carbon Black 10 parts
(CONDUCTEX SC: average particle
size: 20 m.mu., manufactured by
Colombia Carbon Co., Ltd.)
Stearic Acid (for industrial use)
0.2 part
______________________________________
Solvents
______________________________________
Cyclohexanone 50 parts
Methyl Ethyl Ketone 100 parts
Toluene 100 parts
______________________________________
Coating Solution for Upper Layer
Ferromagnetic Particle
______________________________________
Fe--Ni Alloy Particle 100 parts
(Hc = 1600 Oe, S.sub.BET = 40 m.sup.2 /g)
______________________________________
Binders
______________________________________
Vinyl Chloride Copolymer 10 parts
(MR-110: containing an epoxy group
and an --SO.sub.3 Na group, manufactured by
Nippon Zeon Co., Ltd.)
Polyurethane 5 parts
(UR-8300: containing an --SO.sub.3 Na
group, manufactured by Toyobo
Co., Ltd.)
______________________________________
Curing Agent
______________________________________
Polyisocyanate 5 parts
(Coronate L-75: manufactured by
Nippon Polyurethane Co., Ltd.)
______________________________________
Additives
______________________________________
Carbon Black 0.5 part
(Asahi Carbon #50: average
particle size: 90 m.mu., manufactured
by Asahi Carbon Co., Ltd.)
Stearic Acid (for industrial use)
0.2 part
______________________________________
Solvents
______________________________________
Cyclohexanone 50 parts
Methyl Ethyl Ketone 100 parts
Toluene 100 parts
______________________________________
Each of the above-described compositions was kneaded with an open kneader,
and then dispersed by the use of a sand mill to obtain a coating solution
for a lower layer and a coating solution for an upper layer. The resulting
coating solutions were applied to a surface of a polyethylene
terephthalate film (A). Then, magnetic field orientation treatment, drying
and super calender treatment were conducted, followed by slitting to a
width of 8 mm to produce a video tape.
The polyethylene terephthalate film (A) has two layers, a surface on which
the magnetic layers were provided having a surface roughness (Ra) of 6.0
nm, the opposite surface having a surface roughness (Ra) of 15 nm. The
surface layer of the support opposite to the surface on which the magnetic
layers had been provided was subjected to antistatic treatment to give a
surface electric resistance of 1.times.10.sup.11 .OMEGA./sq.
Antistatic agents were added at the following ratio:
______________________________________
Dimethyl Terephthalate 100 parts
Ethylene Glycol 60 parts
Polyethylene Glycol 2 parts
Sodium Dodecylbenzenesulfonate
1 part
______________________________________
The thickness of the support was adjusted to give a ratio of the layer not
subjected to antistatic treatment to the layer subjected thereto of 9/1.
Example 2
The sample was produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy particle having a specific surface
area of 50 m.sup.2 /g was substituted for the ferromagnetic alloy particle
of the upper magnetic layer in Example 1, and that the magnetic layer was
formed as a single layer having a thickness of 2.5 .mu.m.
Example 3
The sample was produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy particle having a specific surface
area of 60 m.sup.2 /g was substituted for the ferromagnetic alloy particle
of the upper magnetic layer in Example 1, and that the thickness of the
support was adjusted to give a ratio of the layer not subjected to
antistatic treatment to the layer subjected thereto of 7/3.
Example 4
The sample was produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy particle having a specific surface
area of 70 m.sup.2 /g was substituted for the ferromagnetic alloy particle
of the upper magnetic layer in Example 1, and that the thickness of the
support was adjusted to give a ratio of the layer not subjected to
antistatic treatment to the layer subjected thereto of 5/5.
Example 5
The sample was produced in the same manner as in Example 1, with the
exception that TiO.sub.2 (diameter: 35 m.mu., specific surface area: 40
m.sup.2 /g) was substituted for Co-.gamma.FeO.sub.x of the solution for
the lower layer in Example 1.
Example 6
The sample was produced in the same manner as in Example 1, with the
exception that acicular .alpha.-Fe.sub.2 O.sub.3 (specific surface area:
50 m.sup.2 /g, acicular ratio: 1/8) was substituted for
Co-.gamma.FeO.sub.x of the solution for the lower layer in Example 1.
Example 7
The sample was produced in the same manner as in Example 6, with the
exception that Ba ferrite was substituted for the ferromagnetic alloy
particle of the upper magnetic layer in Example 6, and that the thickness
of the support was adjusted to give a ratio of the layer not subjected to
antistatic treatment to the layer subjected thereto of 8/2.
Examples 8 and 9
The samples were each produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy powder having a specific surface area
of 50 m.sup.2 /g was substituted for the ferromagnetic alloy powder of the
upper magnetic layer in Example 1, and that the surface properties
(surface roughnesses) of the magnetic layer were changed by varying the
calender treatment temperature.
Example 10
The sample was produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy particle having a specific surface
area of 60 m.sup.2 /g was substituted for the ferromagnetic alloy particle
of the upper magnetic layer in Example 1, and that the thickness of the
support was adjusted to give a ratio of the layer not subjected to
antistatic treatment to the layer subjected thereto of 3/7.
Example 11 and Comparative Example 1
The samples were each produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy particle having a specific surface
area of 50 m.sup.2 /g was substituted for the ferromagnetic alloy particle
of the upper magnetic layer in Example 1, and that the surface properties
(surface roughnesses) of the magnetic layer were changed by varying the
calender treatment temperature.
Comparative Examples 2 to 4
The samples were each produced in the same manner as in Example 1, with the
exception that a ferromagnetic alloy particle having a specific surface
area of 50 m.sup.2 /g was substituted for the ferromagnetic alloy particle
of the upper magnetic layer in Example 1, and that the size and amount of
the filler contained in the surface layer of the support opposite to the
surface on which the magnetic layers were provided were changed to give
surface roughnesses of 3, 35 and 45 nm, respectively.
Measuring Conditions
Surface Electric Resistance:
The surface electric resistance was measured by the use of a digital
surface electric resistance meter (TR-8611A manufactured by Takeda Riken
Co., Ltd.) at 23.degree. C. at 70% RH.
Surface Roughness:
For the samples (Examples 1 to 11 and Comparative Examples 1 to 5), the
center line average roughness in a region of 250.times.250 nm.sup.2 was
measured by the MIRAU method, using a non-contact type surface roughness
tester (TOPO3D manufactured by WYKO Co., LTD.).
Scratches after VTR Running:
The samples produced in Examples and Comparative Examples were each slitted
to a width of 8 mm, and incorporated in a half for an 8-mm video cassette.
Then, 500 running passes were repeated using an 8-mm VTR (FUJIX-M6,
manufactured by Fuji Photo Film Co., Ltd.), and the tape surface (the
surface of the nonmagnetic support opposite to the surface on which the
above-described magnetic layer was provided) after running was observed at
a magnification of from 1.times. to 400.times. under a microscope.
The evaluation was described by using the following marks.
______________________________________
Very good (scratches are little observed)
o
Good .DELTA.
Bad x
______________________________________
Stain of Guide after VTR Running:
The samples produced in Examples and Comparative Examples were each slitted
to a width of 8 mm, and incorporated in a half for an 8-mm video cassette.
Then, 500 running passes were repeated using an 8-mm VTR (FUJIX-M6), and
stain of a stationary guide mounted in the VTR and stain adhered to the
tape surface (the surface of the nonmagnetic support opposite to the
surface on which the above-described magnetic layer was provided) after
running were observed at a magnification of from 1.times. to 400.times.
under a microscope.
The evaluation was described by using the following marks.
______________________________________
Very good (dirt is little observed)
o
Good .DELTA.
Bad x
______________________________________
Dropout after Repeated Running:
The dropout after 500 repeated running passes was measured using a 8-mm
video tape recorder (V-S900 manufactured by Sony Corporation) and
SHIBASOKU VHO1BZ dropout counter (manufactured by Shibasoku Co., Ltd.)
under the conditions of 15 .mu.S and -10 dB.
The evaluation was described by using the following marks.
______________________________________
Very good (less than 20 dropouts/minute)
o
Good (20 to 100 dropouts/minute)
.DELTA.
Bad (more than 100 dropouts/minute)
x
______________________________________
Electromagnetic Characteristics:
The output level at a recording frequency of 7 MHz was measured, using an
8-mm video tape recorder (V-S900, manufactured by Sony Corporation) and
3585A Spectrum Analyzer manufactured by Hewlett Packard. The output level
is shown by a relative value to a value of an 8-mm standard tape
(FUJI-SAG, D6-120, manufactured by Fuji Photo Film Co., Ltd.) taken as 0
dB.
Adhesion:
The samples produced in Examples and Comparative Examples were each slitted
to a width of 8 mm. After adhesive tape was stuck on the surface of the
magnetic layer, the tape samples were fixed to a spring scale. The burden
was measured as the adhesive ability when the tape was torn from the
samples by 180 degrees.
The evaluation was described by using the following marks.
______________________________________
Very good o
Good .DELTA.
Bad x
______________________________________
Results obtained by the above-described evaluating methods are shown in
Table 1.
TABLE 1
__________________________________________________________________________
Inorganic
Magnetic Layer Particle Layer
Support
(Upper Layer) (Lower Layer)
Back Surface
Ferromagnetic Particle
Surface
Inorganic
Layer Surface
BET Roughness
Particle
Consti-
Rs Roughness
Kind (m.sup.2 /g)
(nm) Kind tution
(.OMEGA./sq)
(nm)
__________________________________________________________________________
Example 1
Alloy particle
40 4.2 Iron oxide
9/1 2 .times. 10.sup.11
15
Example 2
Alloy particle
50 4.0 -- 9/1 3 .times. 10.sup.10
20
Example 3
Alloy particle
60 5.5 Iron oxide
7/3 3 .times. 10.sup.10
15
Example 4
Alloy particle
70 6.0 Iron oxide
5/5 2 .times. 10.sup.11
15
Example 5
Alloy particle
60 2.5 TiO.sub.2
9/1 3 .times. 10.sup.10
12
Example 6
Alloy particle
60 3.0 .alpha.-Fe.sub.2 O.sub.3
9/1 2 .times. 10.sup.11
16
Example 7
Ba-Fe 60 8.0 .alpha.-Fe.sub.2 O.sub.3
8/2 2 .times. 10.sup.10
14
Example 8
Alloy particle
50 10 Iron oxide
9/1 5 .times. 10.sup.11
15
Example 9
Alloy particle
50 4.0 Iron oxide
9/1 2 .times. 10.sup.15
5
Example 10
Alloy particle
60 4.5 Iron oxide
3/7 5 .times. 10.sup.9.sup.
20
Example 11
Alloy particle
50 2.5 Iron oxide
9/1 2 .times. 10.sup.11
16
Comparative
Alloy particle
50 12 Iron oxide
9/1 2 .times. 10.sup.11
12
Example 1
Comparative
Alloy particle
50 4.0 Iron oxide
9/1 2 .times. 10.sup.13
3
Example 2
Comparative
Alloy particle
50 5.0 Iron oxide
9/1 8 .times. 10.sup.10
35
Example 3
Comparative
Alloy particle
50 4.5 Iron oxide
9/1 2 .times. 10.sup.11
45
Example 4
__________________________________________________________________________
Dropout
Electromagnetic
after 100
Characteristics
After VTR Running Running
Output
Stain of Guide
Scratch
Edge Damage
Passes
(dB) Adhesion
__________________________________________________________________________
Example 1
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Example 2
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10 2.0 .smallcircle.
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Example 3
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9 2.5 .smallcircle.
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Example 4
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Example 5
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11 4.0 .smallcircle.
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Example 6
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9 3.8 .smallcircle.
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Example 7
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18 3.5 .smallcircle.
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Example 8
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12 1.0 .DELTA.
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Example 9
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24 3.5 .smallcircle.
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Example 10
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18 2.3 .smallcircle.
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Example 11
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15 3.8 .smallcircle.
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Comparative
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Example 1
Comparative
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Example 2
Comparative
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Example 3
Comparative
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Example 4
__________________________________________________________________________
Note:
Alloy particle; Fe:Ni = 99:1; Iron oxide; Co.gamma.FeO.sub.x, x = 1.37.
As is apparent from the results shown in Table 1, the magnetic recording
media according to the present invention obtained in Examples 1 to 12 are
good in stain of the guide and scratches on VTR running, little in dropout
after 500 running passes, and high in output.
As is described above, the magnetic recording medium comprises the
nonmagnetic support having the magnetic layer comprising the ferromagnetic
particle on one surface thereof, wherein the magnetic layer has a surface
roughness of from 2 to 10 nm, the nonmagnetic support has plural layers,
at least one layer of the plural layers comprises the antistatic agent,
and the nonmagnetic support has a surface roughness of from 5 to 30 nm on
the opposite surface to the surface on which the magnetic layer is
provided; and particularly the antistatic agent is at least one of (1)
polyalkylene glycol, (2) sulfonic acid metal salt derivative and (3)
aromatic amine and/or ammonium salt thereof, thereby obtaining the
magnetic recording medium which is excellent in electromagnetic
characteristics, low in the friction coefficient of the back surface,
little in generation of scratches and an abrasion powder due to repeated
running, and low in electrification, leading to little drop out produced
by adhesion of dust, etc. to the medium, and particularly the magnetic
recording medium which is high in recording density and high in
reliability.
While the present invention has been described in detail and with reference
to specific embodiments thereof, it is apparent to one skilled in the art
that various changes and modifications can be made therein without
departing from the spirit and the scope of the present invention.
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